The present invention relates to a novel optical system for a spectrometer or monochromator which is particularly useful in analyzing soft x-ray and extreme ultraviolet regions, 5-1000 angstroms, of the spectrum.
Spectrometers and/or monochromators generally employ a diffraction grating reflecting electromagnetic radiation ranging in wavelength from the infrared to the soft x-ray regions thereof, approximately 10,000 angstroms to 5 angstroms. Early spectrometer instruments used the Rowland grating which was spherical. Such gratings are still in common use. The grooves in the Rowland grating are ruled in equal intervals from each other along the chord of the surface. An entrance slit and the spectrum produced by the Rowland grating lie on a circle having a diameter equal to the radius of the grating curvature. Thus, the grating functions to focus and disperse the light emanating from a source. At wavelengths below approximately 1,000 angstroms, the angle of incidence of such radiation must be high to provide sufficient reflection efficiency. In other words, the grazing angle to the surface of the diffraction grating must be quite low to provide high reflection efficiency. Unfortunately, the Rowland circle geometry governing the Rowland grating requires that the detector employed therewith be positioned at a low grazing angle to the diffracted rays. This oblique detector orientation generally results in low detection efficiency and requires a long detector length to record spectra. In addition, as a monochromator, wavelength selection requires movement of either the entrance slit or exit slit along the Rowland circle. Finally, the spherical grating at grazing incidence results in unuseable spatial resolution perpendicular to the dispersion. In other words, the Rowland grating produces a large degree of astigmatism.
U.S. Pat. No. 4,398,823, issued to Brown et al, discloses a scanning monochromator which maintains a fixed exit slit. However, the Brown "Grasshopper" monochromator requires that the entrance slit and grating be repositioned in a complicated mechanical motion according to the wavelength of the radiation being scanned. The Brown device also exhibits the focal curve from the spherical grating which lies at a grazing angle relative to the principal ray in the Rowland format.
It is desirable to obtain extremely high spectral and spatial resolution when analyzing soft x-ray and extreme ultraviolet radiation as a source. Spectral resolution should equate to resolving powers of 1,000 to 30,000. Further, a spatial resolution of less than one millimeter is typically required. Moreover, electronic detection systems of contemporary design require a normal incidence spectrum orientation. Also, large and immovable light sources such as synchrotron radiation, laser produced plasmas, and the like, and sophisticated detectors require fixed entrance and exit planes. In addition, most spectrometers and monochromators for these radiation sources are used in ultra-high vacuums. Thus, a simple mechanical motion for tuning the wavelength of a spectrometer/monochromator would be a favorable feature.
Articles by Fonck and Kita separately describe holographically fabricated gratings and mechanically ruled gratings, respectively, which possess groove spacings having varying distances from one another. Oblique spectrum orientation produced by the prior art has been eliminated by the use of such gratings. Further, replacing a spherical grating surface by a toroidal surface with a small minor radius removes astigmatism according to the Fonck et al article. However, the modifications reported by Fonck, Nagata, and Kita eliminating oblique spectrum orientation result in degraded spectral resolution. In addition, the systems proposed by Fonck and Kita, as well as the Rowland grating require movable slits or complicated scanning motions to tune the wavelength of the exit spectrum while maintaining high spectral resolution. As prior discussed, these requirements present a substantial disadvantage with modern large and immovable light sources. An article by Monk proposes using converging light with a planar grating having equally spaced grooves or rulings. However, the system by Monk results in considerable astigmatism at such grazing incidence angles. Also, U.S. Pat. No. 2,995,973, issued to Barnes et al, employs a prism encapsulating a grating where the light source converges and strikes the grating at normal incidence angles. Again, this device does not reflect a significant amount of the electromagnetic radiation in the 5-1,000 angstrom range. An article by Murty puts forth the use of a plane grating having hyperbolic rulings, which is useable at normal incidence angles.
U.S. Pat. No. 4,492,466, issued to Aspnes, teaches a cylindrical grating having variable grooved spacing which slides parallel to the rotational symmetry axis for the purposes of focusing the light upon the same. Light entering the Aspnes device is divergent when it strikes the grating surface. The Aspnes device is restricted to use as a monochromator, and to slowly diverging light, such as light produced by synchrotron radiation in the vacuum ultraviolet region.
U.S. Pat. No. 4,192,994 to Kastner teaches the construction of a focusing device with substantially constant spaced grooves on an aspherical surface.
U.S. Pat. No. 4,312,569 to Harada teaches the reduction of aberrations of a single concave grating having varied groove spacings, produced by a mechanical ruling engine, and accepting divergent light.
Recent developments proposed by Hettrick et al propose the use of focusing mirrors with variable groove or line spacing gratings. However, these systems have not been tunable at high spectral resolution. Hettrick et al has also proposed the use of an aspherical telescope mirror to provide a convergent beam used in conjunction with plane reflection gratings. An erect field spectrometer employing plane, varied-spaced gratings at a grazing angle in a beam converging to a point focus was employed by Hettrick et al. In this case, the point-converging beam was provided by an aspherical telescope. Lai et al have proposed the use of an aspherical mirror in combination with an improved curved groove "Type V" holographic grating. U.S. Pat. No. 4,012,843 issued to Harada et al shows a tool for ruling grating grooves which may have varied spacings. Harada et al describes the use of a grazing incidence monochromator having a varied-space plane grating used in divergent light in conjunction with a plane mirror, which may be translated and rotated by a cam mechanism. The grating is also rotated by a sine-bar mechanism.
Kirkpatrick and Baez combined two spherical or cylindrical mirrors, one placed orthogonally relative to the other, to deliver a high resolution image projected on a film or other detector for the study of microscopic objects using x-rays. It should also be noted that Underwood has shown the use of a bent, initially flat glass or metal strip for use as a glancing incidence x-ray optical element.
A spectrometer or monochromator which employs a varied-spaced grating to produce an erect field spectrum and which is easily tuned to a particular wavelength using a simple mechanism would be a great advance in the field of optics.